Since: (i) the father and the mother of Sally may be married with other people, and (ii) the sister or brother relationship only requires to share one parent, we deduce that there is no a definitive answer to this question.
Example: Sally has three brothers, Sally and their brothers have the same mother but a different father, and those brothers have two sisters Sally and Mary, but Mary and Sally are not sisters because they are from different fathers and mothers, hence Sally has no sister.
For those mathematically inclined: Supposing the three brothers are called Bob (to simplify) and the parents are designed by numbers.
FS = father of Sally = 7
MS = mother of Sally = 10
FB = father of Bob = 12
MB = mother of Bod = 10
FM = father of Mary = 12
MM = mother of Mary = 24
Now MS=MB=10 (S and B are brothers), FB=FM=12 (Bob and Mary are brothers), (FS=7)#(FB=12), and (MB=10)#(MM=24). Now S and M are not sisters because their parents {7,10} and {12,24} are disjoint sets.
Edited several times to make the example trivial and fix grammar.
FS = father of Sally = 7
MS = mother of Sally = 10
FB = father of Bob = 12
MB = mother of Bod = 10
FM = father of Mary = 12
MM = mother of Mary = 24
Now MS=MB=10 (S and B are brothers), FB=FM=12 (Bob and Mary are brothers), (FS=7)#(FB=12), and (MB=10)#(MM=24). Now S and M are not sisters because their parents {7,10} and {12,24} are disjoint sets.
Edited several times to make the example trivial and fix grammar.